Voltage tunable filters with microstrip-coupled and suspended coupled lines based on combline topology can be used in diplexers, digital frequency hopping radios, and communication systems. A typical tunable filter based on combline topology comprises a number of coupled resonators, each generally as illustrated in FIG. 1. Each such resonator is characterized by a resonant frequency fo in Hertz (Hz), fo=1/(2π (LoCo)1/2), where Lo is the inductance presented at the end of the transmission line, Co is the capacitance presented at the end of the transmission line, Lo=Zc×tan(2πfol/c)/2πfo, Zc is the characteristic impedance of the transmission line, l is the length of the transmission line, and c is the phase velocity. Co=C1+C2+C3+C4+C5+ . . . Cm, where C1 . . . Cm are switchable capacitances for each coupled resonator to permit small step frequency tuning.
Tunable combline filters may be realized in several ways, including coaxial coupled lines, an example of which is illustrated in FIG. 2. Coaxially coupled line filters are well known in the art and include, for example, the Maxipole and Powerpole filters available from Pole/Zero Corporation, 5558 Union Centre Drive, West Chester, Ohio 45069 (Minipole and Maxipole). U.S. Pat. No. 4,692,724 also describes filters of this general type. Reference is here also made to: published U.S. patent application 2008/0085694; U.S. Pat. No. 5,994,982; and, U.S. Pat. No. 5,923,233. The disclosures of all of the references cited herein are hereby incorporated herein by reference.